The present invention concerns a lift truck comprising a mobile frame, a tiltable boom provided upon it and tilting means for tilting said boom.
In particular, the invention concerns a lift truck for moving and stacking containers, pallets, coils, other goods and heavy objects.
Most lift trucks are equipped with a double or multiple extending mast, mounted at the front and in which a fork carriage can move vertically.
These lift trucks offer the advantage that the fork carriage moves upright and does not carry out a movement as such during this movement, but when they have a large lift height, they necessarily have a large building height, especially due to the high mast. The total height becomes even larger when these lift trucks are transported on a lorry.
Said large building height and thus the headroom can be disadvantageous under a number of circumstances, for example when driving through relatively low passages on factory sites or in warehouses, under bridges or in tunnels on the road or on board of ships.
For a certain lift height, however, it is possible to reduce the building height by increasing the number of parts of the mast, but this makes the construction much more complex, whereas the lift truck becomes more vulnerable and requires more maintenance. The larger number of mast parts also restricts the view of the driver to a large extent.
The mast being situated at the front, i.e. before the front wheels, is not only disadvantageous to the visibility, but also has for a result that the weight of the loads being lifted applies before said front wheels, so that heavy counterweights are required at the back in order to prevent the lift truck from tipping over.
Lift trucks with a tiltable boom avoid a large number of the above-mentioned disadvantages.
A normal boom which pivots around an axle situated in a fixed place, is disadvantageous in that the end of the boom, and as a consequence the fork carriage, carries out a circular movement around said axle.
As the tilting axle is situated at a distance above the ground, whereas the boom can reach the ground with its far end, the far end of the boom will move forward during part of this circular movement. In some cases, if the lift truck is not backed in due time, the lifted load will collide with other objects, which may result in damages.
Also when a load is picked up or placed at a large height, the lift truck, as it is situated more to the back, can be driven too far forward and can collide with loads or such situated at a lower level.
Moreover, the centre of gravity of the whole varies during the lifting or lowering, which may have a negative influence on the stability of the lift truck.
The load may even be situated above the driver""s cabin then, which is quite dangerous.
The far end of the boom can follow a more or less straight course, however, when the boom is telescopic and can slide in and out.
A telescopic boom with a device for sliding in and out is considerably more expensive, however, than one which cannot extend.
Moreover, in case of heavy loads, whereby the telescopic boom must be made relatively heavy, the lower tubular part of the boom which is connected to the frame by means of the hinge pin, must have large dimensions, among others in the vertical height. As this part is situated next to the driver""s cabin, the lateral visibility of the driver will be reduced to zero as a result thereof, and special expensive and vulnerable electronic means such as cameras must be used to repair this visibility.
EP-A-0.630.779, discloses a lift truck with a boom the rear end of which is however fixed to the frame by means of a support which can be moved in relation to the frame in the longitudinal direction of said frame. The boom is pivoted by means of telescopic hydraulic cylinders.
An arm is fixed with one end by a pivot to the boom and with another and by a pivot to a stationary support fixedly mounted on the frame in such way that when the cylinders pivots the boom, the foremost end of the boom follows a vertical path, while the movable support of the rear end is moved.
Due to the arm and the stationary support, this lift truck is relatively heavy and expensive. Moreover, as the hydraulic cylinders have not only to tilt the boom, but indirectly also to move the movable support, so that these cylinders have to be relatively heavy and the lift truck can in practice not lift very heavy loads.
The invention aims a lift truck with a tiltable but not telescopic boom which remedies the above-mentioned disadvantages and which allows its top end to follow an almost vertical path when being tilted and can lift relatively heavy loads.
This aim is reached according to the invention in a lift truck wherein the boom is not fixed directly to the frame, but by means of a support which can be moved in relation to the frame between a rear position and a front position, preferably in the longitudinal direction of said frame, whereby the lift truck comprises a displacement device or means to move said support which are coupled to the tilting device or means, such that, while the tilting means tilt the boom, the displacement means simultaneously move the support of the boom in such a manner that the front end of the boom follows a path which is almost perpendicular in relation to said longitudinal direction of the frame.
As the boom does not have to be telescopic, it may consist of a framework, as a result of which the visibility for the driver is much increased, and the construction can be made lighter.
The support may consist of a carrier which can slide over a guide which is fixed onto the frame.
The tilting means for the boom may contain one or several hydraulic or pneumatic cylinders, as well as one or several electric jacks.
The displacement means for moving the support of the boom in relation to the frame may also contain one or several hydraulic, pneumatic cylinders or electric jacks.
The displacement means for the support and the tilting means for the boom can be coupled by a mechanical coupling containing a follower which is fixed to the boom and which follows a template provided on the frame.
The displacement means and the tilting means may be coupled to one another by means of a coupling containing means for measuring the angle of inclination of the boom or the position of the support, and control means for controlling the displacement means, the tilting means respectively, as a function of the measurement.
The coupling may hereby contain a sensor for measuring the angle of inclination of the boom, as well as a linear sensor for measuring the position of the support in relation to the frame, and an electronic control which, according to a connection which has been put in beforehand, required between the angle of inclination and the position of the support in order to obtain the above-mentioned almost vertical movement of the front end of the boom, as a function of the measured angle of the boom or the measured position of the support, control the displacement means for the support, the tilting means for the boom, respectively.
The angle of inclination can be measured both directly and indirectly, for example by measuring the variation in length of the cylinder in order to tilt the boom, since there is a fixed relation between both. Also the position of the support in relation to the frame can be measured indirectly by measuring the variation in length of the cylinder in order to move the support.
Another possibility consists in that the displacement means and the tilting means are coupled to one another by a coupling containing a template and a follower, whereby the template is fixed in relation to the frame and imposes a specific path onto the follower, and whereby the coupling contains a detection device or means to detect a change in the vertical distance between the follower and a point which is fixed to the boom at a distance from the geometrical tilting axis, in order to control, as a function thereof, the displacement means for the support, so that the above-mentioned change in the vertical distance is compensated.